Key to US Polyergus species

Key to Polyergus species of the United States based on workers, modified from Trager (2013).

1

Gastral tergites with pubescence very sparse or lacking, smooth and often strongly shining; North American, most abundant from Atlantic Coast states west to Mississippi Valley (one species extends west to southern Rocky Mountains) (Polyergus lucidus group) =>2
Gastral tergites with dense pubescence yielding a silky sheen, obscuring the surface of the integument beneath; Eurasian and North American, in North America from western Great Lakes, prairie and Rocky Mountain states and provinces, to Pacific Coast and Mexican mountains (Polyergus rufescens group) => 7

Vertex pilose, ½ VeM usually > 10; head and mesosomal dorsum matte and sides at most feebly shining => 3
- Host Formica dolosa
Vertex less pilose, ½ VeM usually < 8; head and mesosomal dorsum matte or shiny; but if matte, then sparsely pilose => 4
- Host is never Formica dolosa

Scapes and legs shorter, SI < 105, usually < 100, FI < 140; northern species, New England west to the Dakotas => Polyergus sanwaldi

Head of Polyergus sanwaldi worker

Profile of Polyergus sanwaldi worker

Scapes and legs longer, SI > 100, but usually > 110, FI > 140; southern species, North Carolina to Florida, west to Mississippi and southern Missouri => Polyergus longicornis

Head of Polyergus longicornis worker

Profile of Polyergus longicornis worker

Rear portion of head less pilose, ½ VeM usually < 5; head and mesosoma matte or weakly shining on sides => 5
- Host Formica pallidefulva or Formica archboldi
Rear portion of head pilose, ½ VeM usually > 5; all tagmata at least partly shiny => 6
- Host Formica incerta or Formica biophilica

Larger and less pilose species, LI of most specimens > 390; ½ VeM 0–1 (very rarely more); eastern Canada to northern Florida, west to Wisconsin, Colorado, New Mexico => Polyergus montivagus
- Host Formica pallidefulva

Head of Polyergus montivagus worker

Profile of Polyergus montivagus worker

Smaller and slightly more pilose, LI of most specimens < 390; ½ VeM 1–5; Florida => Polyergus oligergus
- Host Formica archboldi

Head of Polyergus oligergus worker

Profile of Polyergus oligergus worker

Smaller, shorter-limbed, SL 1.19–1.36, HFL 1.72–2.04; northeastern, Midwestern, and higher altitude Appalachian distribution => Polyergus lucidus
- Host Formica incerta

Head of Polyergus lucidus worker

Profile of Polyergus lucidus worker

Larger, longer-limbed, SL 1.68–1.79, HFL 2.08–2.44; southern and south central USA and foothill Appalachian distribution => Polyergus ruber
- Host Formica biophilica

Head of Polyergus ruber worker

Profile of Polyergus ruber worker

Vertex lacking pilosity or with ½ VeM < 8, usually < 4; pronotal erect pilosity usually all dorsal => 8
- Hosts in the Formica fusca group, or less often, in the Formica neogagates group, but exclusive of the Formica cinerea complex
Vertex notably pilose, ½ VeM > 6, usually 10–30; pronotum always with some erect setae on sides near lower edges => Polyergus breviceps
- Hosts usually (at least predominantly) Formica montana, Formica canadensis and/or Formica altipetens

Head of Polyergus breviceps worker

Profile of Polyergus breviceps worker

Head and pronotum moderately pilose, ½ VeM < 6 (rarely more), ½ PnM usually > 4; sometimes with infuscated posterior tergites, if gaster appears entirely brown or blackish, ½ PnM > 4 => 9
Head and pronotum non-pilose or sparsely pilose, ½ VeM 0, ½ PnM 0–2; distinctly bicolored, head and mesosoma red, gaster entirely dark brown to blackish; western Great Lakes states to Dakotas, Manitoba, usually in moist or bog forests => Polyergus bicolor
- Host Formica subaenescens (less often Formica neorufibarbis)

Head of Polyergus bicolor worker

Profile of Polyergus bicolor worker

Scapes fall short of vertex corners by a scape width or more, SI < 85; legs shorter, HFI < 120, usually < 110 => 10
- Hosts various, but apparently never Formica moki
Scapes nearly reaching to slightly surpassing vertex corners, SI > 85, usually > 90; legs longer, HFI > 120; coastal hills of southern California and Baja California => Polyergus vinosus
- Host Formica moki

Head of Polyergus vinosus worker

Profile of Polyergus vinosus worker

Head often and especially sides of mesosoma weakly to conspicuously shining; scapes not reaching vertex corners by about 1.5–2 scape widths, scapes distally clavate, SI 65–82, but usually < 77; legs shorter, HFI 99–120 (but usually < 113 and always < 110 in southern Arizona and Mexico); upper elevation conifer forests of Mexican mountains, north to British Columbia, Rocky Mountain and Plains states, east to Mississippi Valley => Polyergus mexicanus
- Hosts are a wide variety of high elevation or northern Formica fusca and Formica neogagates group species

Head of Polyergus mexicanus worker

Profile of Polyergus mexicanus worker

Head and mesosoma typically entirely matte, or at most weakly shining; scapes longer, not reaching vertex corners by 1.5 scape widths or often less, scapes thickened distally, but at most weakly clavate, SI 75–86, usually > 79; legs longer, HFI 108–121, usually > 115; HD, Mexico north to lower elevations of southern Arizona mountains in oak or mesquite woodlands => Polyergus topoffi
- Known hosts are various Mexican, hot climate Formica fusca group species, including Formica gnava, Formica foreliana, Formica subcyanea